This invention relates generally to the testing of semiconductor circuit products such as operational amplifiers, and more particularly the invention relates to a method of determining settling time of such a product.
The response time of an electrical circuit such as a semiconductor integrated circuit in providing an output signal in response to an input signal is an important consideration in many circuit applications. As signal frequencies increase, response time of signal processing circuitry must increase. Thus, circuit products must be characterized during manufacturing testing to determine operating parameters of the circuits. In testing operational amplifiers, for example, the response of an amplifier to a square wave input is determined by the slew rate (slope) and the settling time of an output signal.
Heretofore, sampling voltage trackers have been employed to track an output signal in response to the step input. By using a strobed comparator, the output voltage is measured at increasing periods of time after a reference time (t.sub.0) when the square wave is applied. The falling edge of a precisely delayed latch enable (LE) pulse must be placed at a single point on the output wave form and the voltage read with a digital voltmeter. The reading is stored, the LE is incremented, and another reading is made and stored. The process of repetitively incrementing the LE signal and taking readings is time consuming.
An improved method of determining slew rate is to determine the times after a reference time (t.sub.0) at which the output voltage reaches two different intermediate voltage levels, and dividing the change in voltage by change in time. The technique does not depend on initial voltage and only two measurements are required. The sampling times for measuring the intermediate voltages are readily established using a voltage controlled oscillator for triggering the input step voltage and strobing a voltage amplifier.